Stance and sighting in

Textbook advice is to determine natural point of aim by closing your eyes, lifting the pistol, and adjusting the rear foot if the sights are right or left of the bull.  When I do this from what initially feels like a comfortable stance the sights are typically a bit to the right.  Adjusting position puts me a small angle off of parallel (shoulders - arm - pistol - lined up with target: perpendicular to the line).  This is the typical stance in videos of 10M air pistol shooters and AMU BE team, which seems odd since most of them are 30-40 years younger and don't have stiff necks yet.

Often my groups are left of center and I wonder if my NPA is really more open (angled from parallel), and experimenting by turning right a bit seems to help. But this also raises the question of whether the pistols are correctly zeroed.

Early on I tried sighting in by resting my wrists on a bag on the bench, but shooting standing the zero seemed off and I changed it. 

So the main question is should you sight-in benched or standing?

Sight in benched, but rest the arm only.  The wrist and gun is not constrained.

It seems to me that you're final zero should be done in the manner that you plan on shooting it.

Use a ransom rest to determine if your barrel/load is inherently accurate. Use the bench to see how the accuracy is affected by recoil (arm only supported like wobbley notes; I tend to get close to wrist). Use standing to see how practical precision affects it. There are a number of different factors at each stage (yes even lighting and temperature). Groups to the left usually tell me I'm not pulling my trigger straight enough back. Also if your arm muscles are tensed differently that may affect your finger. This is the journey. If you can get things worked out in standing position,  great. You might even see some differences on a "bad day" Consistency is #1.

Unfortunately no magic answer exists because all shooters are different and more due to age and physical ailments. Just have to take a position and practice it, if you can make it repeatable for every shot string then that's a good start. Take position, don't look at the target, lift pistol up towards target, then turn head back to the pistol to see where it's pointed, adjust rear foot as needed and repeat. Ideal is to bring your arm up where you aren't forcing it to the left or right to hold your target. In a book or two and shooting in general for some disciplines , if the body is stretched/tensed then it restricts unwanted movement past that point. As for sighting in, mostly one have stretched out on a bench while talking a knee for body alignment.

NPA is far less important in pistol than in rifle. It is more likely that groups in one direction are the result of trigger / grip issues rather than position. Position should be optimized for stability and be comfortable enough to use all day without pain or fatigue.

I am not consistent enough to raise the pistol and check my sight alignment (I broke my collarbone twice and am no longer symmetrical), so I aim at the target, move the pistol a bit with my eyes closed to see if it comes back to center, then make any stance adjustments. I think it is worth some points and saw improvement in scores after an USAMU clinic reminded me to do this.

I can call about 4 out of every 5 shots, so I just adjust the sights while I am shooting. A friend did not believe I could do this so he rested my pistol and the sights were maybe a click off. I actually like sighting in firearms, except for when a rifle scope gets loose in the mounting rings.

At the range yesterday I tried re-sighting from the bench seated on a chair: H208s / Vortex venom dot, resting arms on Caldwell shooting bag.  Kind of surprised that I wasn't much steadier on the bag than standing and shooting one handed - I probably need to practice from the bag.  After centering the groups from the bench I shot groups of 20 from standing, which surprisingly were consistently left and low.  I re-zeroed standing and everything was good again.

Conclusion: for me at this stage I will stick with standing zeroes

Zero from the position you use for the stage. For a good AR or a bolt gun a couple of shots will due on a nice calm day. A sloppy AK might take 20 shots to really know what's going on. The point is, if there is less accuracy, either due to the gun or the person holding it, more shots are needed to get a good "average". 
For a pistol I like to shoot several targets, maybe even over a couple of sessions. Leave the first target in place and staple the new targets over it. After you feel like you have enough shots, adjust to the first target that has a bunch of holes in it. 

Best,
Arthur

I’ve found that NPA drifts throughout a 2700 or long training session. Start with a comfortable stance and try adjusting grip to find your NPA. Once you have it, Find a way to check it and make small movements with the back foot to correct it. I like to use eyes closed, extend arm to where target should be, open eyes. Usually I’m getting into stance and checking NPA while the range is still cold. For me, NPA is pretty important.
I’ve also found that zero from a bench is different from zero offhand for me. Likely something to do with line of sight. Sights have knobs for a reason. Don’t be afraid to use them.

When you're adjusting your aim, it's better to do it while standing to make sure it matches how you naturally hold the gun and to fix any alignment problems you might have when shooting.